Abstract

We exploit 55 repeating microearthquake sequences on the San Andreas Fault, just south of the rupture zone of the 1989 MW 6.9 Loma Prieta Earthquake, to search for time-dependent properties of the Earth's crust. Using moving window waveform cross correlation, we identify clear and systematic delays as large as 20 msec for the direct S wave and exceeding 50 msec in the early S-wave coda following the Loma Prieta mainshock. Others have also identified phase delays (velocity reductions) associated with damaging earthquakes and they have suggested a myriad of possible causal mechanisms. Here, we present new evidence for a mechanism to produce velocity reductions correlated in time and space with an earthquake. A strong correlation between the spatial patterns of S delays and the intensity of strong ground motion in the Loma Prieta Earthquake suggests that physical damage, the formation or growth of cracks during strong ground motion, to the Earth's shallow crust is responsible for the observed velocity reductions. The strong spatial variability in S delays over short distances and the strong correlation of the magnitude of delays with surface geology indicate that the phase delays accumulate primarily near the receiver. The effect is stronger at stations on young, soft rocks than at stations on old, hard rock. Disproportionately larger S coda delays than P coda delays suggest that the cracks formed by the strong shaking are fluid filled. In the 10 years after Loma Prieta, the initial increase in travel times reduces logarithmically with respect to time, often back to the premainshock levels. We attribute this behavior to the same “slow dynamic” healing observed in laboratory studies of the recovery of materials from transient nonlinear strain.

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